fix(codex): cache multi-agent spawn-agent model data and invalidate on updates

- Add registry generation tracking and a Codex catalog revision accessor to provide stable cache keys.
- Cache parsed model templates and rendered spawn-agent markdown, and reuse them across requests when revision/generation are unchanged.
- Invalidate/recompute caches when model registrations or catalog content change, and separate rewrite handling for spawn tool descriptions vs. message encryption stripping.

Closes: #4967